Two-component developers used in electrophotography comprise a toner and a carrier, and the carrier is used for the purpose of imparting a proper amount of polar frictional electricity to the toner.
As such a carrier, a resin coated carrier having a resin coating layer on the core particle surface is used for various reasons including enhancement of the frictional electricity providing capability.
It is known to skilled persons of the art that a resin coating layer comprising a fluorine-containing resin has favorable properties, and that a fluorinated acrylate polymer is particularly preferred as a material for the coating layer, because (1) this polymer has a glass transition point higher than that of other fluorine-containing resins and, therefore, are less liable to melt and stick to the surface of carriers, and (2) electrification can be easily controlled by varying the number of fluorine atoms contained in the branched chain.
As a resin-coated carrier using such a fluorinated acrylate polymer, Japanese Pat. O.P.I. Pub. No. 235964/1988 discloses an electrostatic image developing carrier prepared by coating the surface of core particles with fine particles of a fluorinated acrylate polymer by use of a dry process.
However, these electrostatic image developing carriers have the problems described below.
(1) Resins comprising fluorinated acrylates are generally poor in adhesion to the core particles and low in film forming property. Accordingly, when used in forming a resin coating layer by a dry process, fine particles of these resins cannot be adhered and extended properly to the surface of core particles, yielding a carrier of poor durability. PA1 (2) Fine resin particles used to form a resin coating layer can be prepared by, for example, emulsion polymerization. According to a study of the present inventors, however, resin fine particles obtained by the usual emulsion polymerization do not always form a satisfactory resin coating layer and often give a carrier poor in capability of providing a toner with an adequate frictional electricity. When such a toner is used, formation of sharp images is prevented and, moreover, troubles such as fogging and scattering of toner are apt to be caused in the process of image formation.